Ethylene-propylene-diene terpolymer (EPDM) is extensively used in a variety of applications. For example, it is particularly useful as a polymeric sheeting material, which, because of its excellent physical properties, flexibility, weathering resistance, low temperature properties and heat aging resistance, has gained acceptance as a roofing membrane for covering industrial and commercial roofs. These roofing membranes are typically applied to the roof surface in a vulcanized or cured state and serve as an effective barrier to prevent the penetration of moisture to the covered roof.
Traditional EPDM roofing membranes are generally black or very dark in color, and are typically prepared by compounding the base polymer of EPDM with one or more carbon black fillers, processing oil, and other desired ingredients such as plasticizers, antidegradants, adhesive-enhancing promoters, etc., in a suitable mixer, and calendering the resulting compound into the desired thickness. The roofing membrane may also be cured by vulcanizing the resultant sheet in the presence of one or more vulcanizing agents and/or compatible vulcanizing accelerators. Vulcanizing agents such as sulfur or sulfur-donating compounds such as mercaptans are typically used, although vulcanization and curing may be done using other agents or in the presence of other compounds.
While black or dark-colored EPDM membranes have been used for decades as commercial single-ply roofing membranes, such membranes are known to absorb sunlight and become hot. This has lead to various environmental and energy concerns. In recent years, great efforts have been made toward producing white and/or other non-black membranes, including not only EPDM membranes but also thermoplastic olefin (TPO) membranes, that are more environmentally friendly and more energy efficient. Various mineral fillers such as clay, talc, silicas, mica, calcium carbonate, and the like, either individually or in various combinations, have been added to various roofing membrane formulations to improve energy absorption characteristics.
Because non-black or light colored membranes do not have black pigments or carbon black in them to absorb UV light, UV stabilizers have been needed in the membranes to prevent the polymers from degrading in the sunlight. UV light stabilizers have been used in various TPO formulations (TPO is a thermoplastic material) where the formulations are to be used in membranes for outdoor use. For instance, a typical UV light stabilizer for plastic products, including TPO membranes, is a hindered amine light stabilizer, also referred to as HALS. HALS acts as a radical scavenger. Through the formation of nitroxyl radicals, hindered amines terminate and deactivate any alkyl radicals and/or peroxide radicals that may participate in the photo-oxidation process.
Unfortunately, due to the strong basic nature of the hindered amines, HALS cannot be used in thermoplastic roof membranes that are acidic in nature, such as polyvinyl chloride (PVC) membranes. It is well known that PVC releases hydrochloric acid (HCl) upon thermal and photo-oxidation. It is believed that the HCl combines with HALS and obstructs the stabilizer's ability to form nitroxyl radicals, thereby impairing the scavenger ability of the HALS. It is further believed that membranes containing EPDM rubber in their formulations could not use HALS due to similar, but not identical, reasons as those above. It is believed that HALS reacts with one or more ingredients, e.g., a filler, a curative, the EPDM rubber, during the rubber mixing and/or curing processes, the reaction causing the HALS to not form the necessary nitroxyl radicals, due at least in part to the high-temperature reactive environment.
Nevertheless, the use of HALS as a radical scavenger and UV light stabilizer would be beneficial in non-black EPDM membranes, if it would not be deactivated by acids or by the high-temperature processes as required in the production of EPDM membranes. Thus, the need exists for a HALS-based UV light stabilizer suitable for use in a non-black EPDM membrane and, more specifically in one embodiment, a sulfur-cured EPDM membrane.